1. Field of the Invention
This invention relates in general to improvements in valve gate actuators used in injection molding systems. Although not limited to any particular field of use, the actuator of the present invention is in particular useful for the fabrication of large molded parts such as, for example, automotive bumper fascia.
2. Description of the Related Art
In hot runner systems used in injection molding, valve pins are used to open and close the gate to a cavity in the mold in which the molded part is formed. Typically, the valve pin is opened prior to the start of injection allowing plastic to flow into the cavity of the mold. The valve pin is maintained in the open position while the melt material is being packed in the cavity to form the molded part. At the end of packing, the pin is closed to eliminate any drooling from the hot runner nozzle and eliminate any vestige that would be left on the molded part by forming a smooth surface with the inner surface of the mold cavity.
The valve pin is typically located in the center of the bore of the hot runner nozzle. In a multiple gate system that uses a plurality of nozzles, the nozzles typically will be connected to a heated manifold, which in turn receives the machine nozzle from the injection molding machine. The tip of the valve pin extends to the gate of the mold. From its tip, the valve pin extends through the hot runner nozzle, through a bore in the manifold, and is connected at one end to an actuator located above the manifold and attached to a top clamp plate. A valve pin cylinder in the actuator is usually actuated using either hydraulic or pneumatic pressure.
In hot runner systems, that there are two basic techniques for providing valve pin actuators. The first technique is to build the hydraulic actuator assembly into the top clamp plate. With this product, when performing maintenance on, or disassembling, the system, it is necessary to allow the manifold to cool, then remove the valve pin from the hot runner manifold prior to removal of the top clamp plate. This can require a significant amount of time. Since the valve pin is precision machined to fit at the gate, great care must be taken when replacing the valve pin. Molds for large automotive parts are very large, thus, ease of assembly and disassembly for maintenance of the hot runner is an important issue.
A further drawback of prior systems is the fact that the valve pin itself is typically mounted in the actuator in a relatively fixed position and is free to move only in one direction. The manifold will expand relative to the top clamp plate when heated. Thus, the valve pin actuator has to be positioned so that the pin can slip relative to the actuator in the expansion direction. If the alignment or predicted expansion direction is off, the pin sees a side load resulting from the expansion of the manifold, possibly resulting in binding of the pin in the valve pin bushing, or even pin breakage. Thus, prediction of the line of expansion is necessary along with precise alignment of the valve pin.
Another type of actuator has the entire hydraulic actuator assembly bolted to the hot runner manifold. This system includes through holes in the top clamp plate to provide clearance for the actuator assembly. There are several drawbacks associated with this system. For example, because the actuator cylinder is directly bolted to the heated manifold, the actuator cylinder needs water channels formed therein for cooling. Further, because it is bolted directly to the manifold, flexible water and hydraulic lines must be run to each valve gate actuator between the manifold and the top clamp plate. It can be cumbersome to position these lines, and they can get hot due to proximity to the manifold causing deterioration of hoses and fittings. Leakage can result. If hydraulic fluid contacts the hot manifold, hazards may result.
Accordingly, it is an object of the present invention to provide an improved valve pin actuator, particularly wherein the actuator itself can be at least partially disassembled without requiring removal of the valve pin.
Another object of the present invention is to provide a valve pin actuator in which the cylinder and piston along with the hydraulic fluid lines are retained within the top clamp plate, while the valve pin assembly itself stays with the hot runner system.
Still another object of the present invention is to provide an improved valve pin actuator wherein the actuator can be at least partially disassembled without the need to drain the hydraulic fluid lines, and without the need to remove the valve pin from the hot runner manifold.
A further object of the present invention is to provide an improved valve pin actuator that provides for a clearance between the actuator piston and actuator cap. This clearance allows for relative expansion between the hot runner manifold and the top clamp plate in any direction, without putting a significant side load force on the valve pin.
Still another object of the present invention is to provide an improved valve pin actuator that provides for a more simplified operating hydraulic circuit.
In one illustrative embodiment of the invention, an injection molding system is provided that includes a valve pin actuator adapted for mounting between a plastic distribution manifold and an overlying clamping plate. The valve pin is adapted to extend through an injection nozzle and positioned to seat and unseat at a mold gate.
The valve pin actuator includes a cylinder mounted to the clamping plate; a piston slidably mounted in the cylinder; a valve pin assembly carried by the piston and for holding a top end of the valve pin; and a circuit coupled to said piston to control sliding movement thereof and in turn translation of said valve pin between the seated and unseated positions.
The valve pin assembly includes a first part removably secured to the piston so as to translate therewith, and a second part for receiving said valve pin and secured to said manifold.